Your syntax is correct. It seems that you actually need to call resolve() as the error message says, to resolve substitutions. I guess in 2.0.x the play framework did this and provided a config that was already resolved this way. Now that the config API is used directly it needs to be resolved manually.
Add a call to resolve()
in this line:
val conf = ConfigFactory.parseFile(new File("conf/application.conf")).resolve()